I think sciences are generally straightforward. Maths and languages are very similar on how they work, but languages are probably a bit more harder since you have to use other skills as well. Music is also similar and just because some can play an instrument brilliantly doesn't mean they can succeed in this subject. History is demanding in exams according to my friends and the sheer amount of content needed to do well is rather shocking. Art finally requires a lot of talent and after watching my friend, the workload is surprisingly high.

it depends how your mind works
for some people, maths/science is easy while the humanities/english subjects are difficult
as for me, I am god-awful at maths (and science) but I am all right at the humanities/englishes
but usually, the humanities are "easier" relatively in comparison to the mathematical subjects so I'd say things like physics, maths and chemistry are the hardest